1. Field of the Invention
The disclosure generally relates to a die attach composition used for bonding a silicon chip on a flat substrate, and more particularly, to a die attach composition providing accurate placement of heater chip on a flat ceramic substrate when manufacturing an inkjet printhead assembly.
2. Description of the Related Art
A printhead assembly, employed in an inkjet printer, generally includes a silicon chip used as a heater chip, a nozzle plate attached on the silicon chip, and a flexible circuit. The printhead assembly is typically formed by attaching the silicon heater chip with nozzle plate on a chip pocket of a printhead body using a die bond adhesive. The silicon heater chip is then connected to the flexible circuit using a technique commonly known as the tape automated bonding (TAB) technique. The printhead body may be a polymeric material such as a modified phenylene oxide.
Alternatively, the printhead assembly may also be formed by separately attaching the flexible circuit and silicon heater chip with nozzle plate on a flat substrate such as a flat ceramic substrate instead of on the chip pocket of the printhead body. The heater chips are then connected to the flexible circuit through wire bonding technique instead of using the TAB technique. The silicon heater chip is attached on the flat ceramic substrate by disposing die bond adhesive on the flat ceramic substrate, placing the silicon heater chip over the disposed die bond adhesive without any support from the TAB circuit, and then curing the die bond adhesive.
This above-described alternate way of forming a printhead assembly using a flat ceramic substrate presents a problem because there is no TAB circuit for the heater chip to rest upon. The absence of the TAB circuit causes the silicon heater chip to shift position after the die bond adhesive is cured, obviously leading to a faulty printhead assembly. Thus, there is a need for a die attach composition having improved thixotropic properties to provide accurate placement of silicon chips on flat substrate, especially a ceramic substrate.
There are many commercially known die attach adhesive which can accurately bond and position silicon chips on flat substrate, However, these adhesives tend to wick and easily wear upon exposure to aqueous fluid or ink. In the construction of printhead assembly of an inkjet printer, it is desirable to employ a die attach composition having improved thixotropic properties to provide accurate placement of heater chips on flat ceramic substrate while also possessing good ink resistance.
In pending patent application U.S. patent application Ser. No. 12/103,307 assigned Lexmark International, Inc, there is disclosed an encapsulant composition comprising from about 1.5 to about 95 percent by weight of one or more cross-linkable epoxy resins having rigid backbone and about 0.1 to about 35 percent by weight of one or more thermal curative agents. This prior art encapsulant composition exhibits a rigid and strong adherence to the surfaces of the printhead of an inkjet printer and a high ink resistance when the silicon heater chip is connected to the flexible circuit using the TAB technique. However, this prior art encapsulant composition cannot be used as die attach adhesive when the printhead includes a flat ceramic or plastic substrate because the silicon heater chip does not position correctly on the flat substrate after the curing process. The absence of the TAB circuit causes the silicon heater chip to shift position after the die bond adhesive is cured. This misalignment of the heater chip and the flat substrate leads to a faulty printhead assembly.
In the present invention, this prior art encapsulant composition is modified to have surprisingly improved thixotropic properties and provide a die attach composition which allows accurate placement of silicon chips on flat ceramic substrate while also exhibiting high ink resistance. This provides a printhead assembly having all its components properly aligned on the flat ceramic substrate, therefore leading inkjet printers having good print quality.